1. Field of the Invention
The present invention generally relates to a quantum computer. Specifically, a quantum bit (qubit) based on a gradiometer superconducting flux qubit design provides significant noise immunity and two independent input controls, one each for Sx and Sz fields.
2. Description of the Related Art
Relative to classical computers, a quantum computer potentially offers an enormous gain in the use of computational resources, including time and memory. Classical computers need exponentially more time or memory to match the computational power of a quantum computer when appropriate problems are addressed.
Experimental and theoretical research in quantum computation is accelerating world-wide. New technologies for realizing quantum computers have been proposed and continue to be further analyzed and improved.
The basic unit of quantum information in a quantum computer is a quantum two-state system, called a “quantum bit” (“qubit”). A qubit is a superposition of its two logical states 0 and 1. Thus, a qubit can encode, at a given moment of time, both 0 and 1.
An ideal hardware implementation of the qubit should be: 1) a controllable high-coherence (e.g., Q-factor, the time for which the wavefunction remains quantum-coherent, per unit time required to implement a qubit operation—of at least 105) quantum 2-level system, and 2) scalable (i.e., many qubits, for example, 104, can be manufactured and operated cheaply).
A key element in the search for practical quantum computer designs is finding an improved hardware implementation of the qubit. After successes with few-qubit systems, including demonstration of the Schor factorization algorithm with NMR (Nuclear Magnetic Resonance)-based techniques, further progress awaits development of scalable qubits. For example, existing qubit implementations (such as by NMR) have achieved limited success (such as demonstrating factorization of 15), but have run into limitations of non-scalability.
Using lithography, for example, manufacture of the thousands of similar qubits required in a practical quantum computer becomes feasible. One scalable approach being explored implements the qubit as a micron-scale superconducting circuit. Recently, superconducting implementations with a long coherence lifetime, approaching that required for realistic quantum computation, have been demonstrated.
For example, a type of superconducting Josephson-junction qubit has recently been shown to have a Q-factor of order 104, which approaches that required in a quantum computer. Such qubits can be cheaply made in multiple copies on a chip by lithography, and are, therefore, scalable. However, the approach described is a charge qubit, whose states are defined in terms of the presence or absence of a single electron-pair, and, therefore, is likely to lack robustness for a commercial environment.
Thus, the conventional superconducting qubits have either involved a nanoscopic quantum dot, whose bistable state is defined by the presence/absence of a single electron pair, or operate in an intermediate regime where the defined state is a hybrid of charge and flux (sometimes termed a ‘phase’ qubit).